1. Field of the Invention
The present invention relates to a sensor apparatus disconnection detecting circuit comprising a control circuit and a sensor circuit including a functional circuit having one or a plurality of transistors for outputting a sensor signal to the control circuit upon receipt of power supply from the control circuit in a state where a plurality of connection lines and terminals are interposed between the control circuit and the sensor circuit, with the disconnection detecting circuit being made to detect a disconnection or breakage of at least one of the connection lines.
2. Description of the Related Art
So far, for example, in a sensor apparatus mounted in a vehicle, a sensor circuit and a control circuit (for example, ECU) have been connected to each other through a plurality of connection lines (for example, power supply lines, sensor signal lines and ground connection lines) and terminals Vcc, Vout, GND and others.
In FIGS. 6A and 6B schematically showing an interface section, a control circuit 1 and a sensor circuit 2 are connected to each other through a plurality of connection lines 3 to 5 (a power supply line 3, a sensor signal line 4, a ground connection line 5). In this configuration, a circuit is provided to detect the connection conditions therebetween in case of disconnection of at least one of these connection lines 3 to 5 for some reason.
On the control circuit 1 side, for the detection of disconnection of the connection lines 3 to 5 between the control circuit 1 and the sensor circuit 2, as shown in FIGS. 6A and 6B, a pull-up resistor 6 (or a pull-down resistor 7) is connected to a power supply circuit 8 to be placed in parallel in an internal terminal of the control circuit 1 to set the ratio of the impedances (that is, the pull-up resistor 6 or the pull-down resistor 7 and a resistance value R0) of the control circuit 1 and the sensor circuit 2 to an extreme value (approximately 15 to 20:1).
In this arrangement, in a case in which at least one (for example, the sensor signal line 4) of the connection lines 3 to 5 falls into a disconnected condition for some reason, a voltage to be applied to a terminal Vin on the control circuit 1 side approaches a power supply voltage (or a ground voltage) on the basis of a variation of current flowing in the pull-up resistor 6 (or pull-down resistor 7). Accordingly, if this power supply voltage (or the ground voltage) is set out of an input voltage range in a normal operation of an internal circuit 9 of the control circuit 1, the internal circuit 9 of the control circuit 1 can detect this voltage to detect the disconnection of at least one of the connection lines 3 to 5. As one example of disconnection detection on this principle, there has been known a disconnection detecting apparatus disclosed in Japanese Patent laid-Open No. HEI 5-107292.
Meanwhile, so far, a gold-plate-treated terminal has been employed as a terminal to be used for the electrical connections between the control circuit 1 and the sensor circuit 2 through the connection lines 3 to 5 and, in this case, in FIGS. 6A and 6B, the disconnection detection is made in a manner such that the value of the pull-up resistor 6 is set at approximately several hundreds kΩ and the resistance value R0 of a resistance element connected to an internal side of a terminal of the sensor circuit 2 is adjusted to produce an impedance for an impedance ratio (approximately 15 to 20:1) relative to the first-mentioned resistance value.
However, according to the recent required specifications, a low-priced terminal tin-plate-treated (which will be referred to as hereinafter as a “tin-plate terminal), in place of the gold-plate treatment, has also been put to use. On the other hand, this raises fears that the contact resistance increases with the deterioration of the contacts of the tin-plate terminal and, hence, there is a need to increase the energizing current to the tin-plate terminal in a normal operation in a manner such that the value of the pull-up resistor 6 is made lower than before to be in a range between several kΩ and several tens kΩ. If the value of the pull-up resistor 6 on the control circuit 1 side is set at several kΩ to several tens kΩ, a difference of approximately one to two digits appears with respect to the conventional case, which creates a problem even if, as well as the conventional case, the change of the resistance value R0 is made to detect a disconnection while meeting that specification.
That is, for the impedance of the sensor circuit 2 at the occurrence of a disconnection to be extremely lower than the value of the pull-up resistor 6 (or pull-down resistor 7) on the control circuit 1 side, for example, as compared to the conventional case, there is a need to further decrease the value R0 (impedance) of a resistor connected in parallel to an internal side of the sensor circuit 2 side terminal. However, since the current flowing in the resistor connected to this terminal increases considerably due to the relationship between resistance values in a normal operation, it leads undesirably to increasing the useless current flowing in that resistor R0.